Brick and stone masonry have served as reliable and valuable elements of commercial building projects for centuries, gracing urban and rural landscapes since time immemorial. Building Teams have trusted clay brick to bring durability and sustainable qualities as well as good performance in moisture and thermal protection to their projects. More recently, brick veneers have begun to offer attractive structural potential with high strength-to-weight ratios, and masonry veneers are available in literally thousands of configurations and looks.
Building Teams have been taking advantage of both the timeless aesthetics and the modern capabilities unique to stone composites, concrete block, and manufactured stone, as well as masonry veneers. Each offers a unique look and performance benefits.
As old and new masonry choices continue to appeal to building design and construction professionals, this may be a good time to undertake an overview of the different options and their associated benefits and drawbacks.
When choosing an exterior cladding finish—whether brick veneer, marble or granite composite, precast concrete, or slate—Building Teams should consider options for each material or system based on multiple system benefits, such as greater insulation values, reduced maintenance requirements, and product enhancements that improve durability.
BEARING WITNESS TO MATERIAL CONSIDERATIONS
When considering brick and masonry materials, it is important to recognize that each material possesses its own inherent characteristics that are important to evaluate for each project application. For example, brick ranks high when it comes to aesthetics and durability. With a life expectancy of 75-100 years, brick is a highly sustainable material; it is also 100% recyclable and offers what many consider a classic visual quality to the façade that is unlikely to go out of fashion.
“With hundreds of brick colors and a wide variety of textures, combinations can be from smooth hospital white walls to warm red and brown tones reminiscent of a colonial mansion,” says Brian E. Trimble, PE, LEED AP, a regional vice president in engineering services and architectural outreach with the Brick Industry Association, Reston, Va., a trade group representing brick manufacturers. And because it is laid in the field, brick—like other unit masonry products—works well with uniquely shaped structures.
After reading this article, you should be able to:
- Compare materials and systems used to construct commercial masonry wall assemblies for optimal sustainability.
- Understand the various kinds of backup wall assemblies and how they contribute to environmentally efficient buildings.
- Describe the difference between barrier and rainscreen walls using masonry and brick veneer and their respective environmental and human health benefits.
- Discuss the key detailing and specification criteria for functional and effective masonry wall systems to prevent moisture and air penetration.
Another option for Building Teams to consider: marble and granite composite panels. These are relatively costly compared to brick but may be worth considering when the project demands the inimitable look of stone. Composite panels deliver the aesthetics without the associated weight of stone, often allowing some cost savings on the structural package. The panels arrive at the job site in dimensions of up to 5x10 feet. At the same time, says Bruno P. Gubetta, president of Alpine Building Restoration, Waterbury, Vt., “There is a high energy use required to produce these panels, they are not recyclable, and there is no long-term history of use to observe performance.” However, other building professionals consulted for this article state that many composites have a long track record of performance.
Looking at natural options, granite is an extremely hard rock, very durable and resistant to weathering. By contrast, the surface of marble will eventually crystallize when exposed to the sun, which can cause warping of veneer panels. When applied as a thick veneer, however, marble is not as prone to crystallization. Marble is also easy to carve and cut, making it ideal for decorative exterior applications. Then there is slate, which is most commonly used as a roofing material due to its moisture-absorbing qualities; however, it is occasionally used on façades for decorative rainscreens.
A decidedly modern alternative to natural stone, thin precast concrete comes in a range of tinted colors and textures and is seen as a practical application for certain large low-rise buildings and industrial facilities. “Precast concrete used in a veneer curtain wall can be a beautiful material and is less expensive than granite and marble,” says Roger Hartung, AIA, NCARB, a principal with the Pittsburgh-based architecture firm IKM. “At the same time, I have found its quality can vary widely.”
Because precast concrete panels are highly engineered and created in a controlled manufacturing environment, it is reasonable to expect that they would exhibit high levels of quality assurance. However, installation errors can quickly compromise performance. To counter this, IKM always requires mockups of the exterior envelope, with careful attention to detail and installation practices during construction.
USING BACKUP WALLS EFFECTIVELY
Another quality that all these veneer types have in common is that they require a backup wall for structural integrity. When choosing between substructure options such as concrete masonry units, cast concrete, and steel, it’s important to carefully weigh the pros and cons.
Using brick veneer with a CMU backup is the Rolls-Royce of wall systems in the opinion of Alpine Building Restoration’s Gubetta, who has worked as a master mason for the past 33 years. “This combination is indestructible, easy to insulate, 100% recyclable, offers low embodied energy, and can be either load-bearing or non-load-bearing. It’s also economical, and the walls last forever.”
Largely seconding that opinion is Edward Gerns, RA, LEED AP, a principal in the Chicago office of Wiss, Janney, Elstner Associates. “When designed and installed properly, this system is very durable and provides excellent water management, thermal mass, and good aesthetics,” says Gerns. who specializes in façade inspection and exterior wall evaluation. He differs with Gubetta on one point—that brick-with-CMU is the most economical system. He suggests that when considered over the service life of a building, the actual expense is comparable to other systems.
When erecting a brick-over-CMU system, mortar used during construction can end up blocking air gaps, weeps, and brick vents, so it’s important to make sure that air movement is maintained within the cavity system, according to Robert M. Donaldson, LEED AP BD+C, NCARB, an architect with Bostwick Design Partnership, Cleveland.
While brick-over-precast concrete is generally a lighter system than brick-over-CMU, Gubetta notes that it is usually more difficult to insulate and more expensive to install because two trades must be involved. Brick over CMU generally requires a single subcontractor trade.
An even lighter option is brick over steel stud, which can also often be constructed in the shortest time frame. Although it’s a very common commercial wall system, it does require additional infrastructure to make it a complete wall cavity.
When working with a concrete block veneer, a concrete block backup wall is an economical option, as the materials and labor can be relegated to a single mason. However, a double-wythe concrete block wall may be limited in the height at which it is self-supporting, as it requires reinforcement for stability, according to Donaldson, who has worked on more than 100 masonry projects, including a few that have received awards from the Masonry Institute. He notes, too, that “a concrete block backup is limited in its structural capacity, not just for the concrete block veneer component, but also for the loads exerted on the block from above.”
By using a steel backup, the Building Team can open up the potential for reaching greater heights. On the other hand, steel-based systems run a higher risk of corrosion over time, and they are generally more costly.
When choosing a stone slab façade (generally defined as a 1.25-inch-thick cladding), Gubetta recommends a CMU backup wall because this provides the stone panel installer with unlimited anchoring attachment points. Gubetta also prefers CMU over cast concrete because CMU is easy to install and insulate and offers design flexibility at a good price point.
Yet another variation is using stone veneer on reinforced concrete block, which Gubetta considers to be another superior wall system. “It is the most tried-and-true system, requires only one trade, and is the best value for the dollar,” he says. According to Donaldson, the veneer joints are set with slips attached to the block, but they must be flexible and used with sealant. This way, the stone is allowed to move on the clip system without the adjacent stone panels impacting each other.
BARRIER VS. CAVITY WALLS
Once the veneer and backup wall are selected, a number of detailed construction directives must be carefully heeded to ensure a tight, high-performance wall system. For starters, specifiers must decide between a cavity wall and single-wythe barrier wall, even though the latter is seldom used.
“We rarely use a single-wythe wall anymore except on the rear walls of a strip commercial project. And even then, the details must be carefully coordinated with the structural engineer,” says Christian Rogers, AIA, LEED AP, with Blackmon Rogers Architects, Mountain Brook, Ala.
To perform properly, barrier walls must be very thick, which increases their expense, and they are hard to insulate. In addition, the single-wythe wall is much more susceptible to water penetration. Although flashing can be installed, the reinforcing used within the block often interferes, thereby making its application rather problematic.
On the other hand, the cavity wall provides several lines of defense against moisture penetration, is easy to insulate, and can act as a structural wall capable of bearing loads or shear forces, says Hartung, a 25-year veteran of the industry who has presented at the International Masonry Institute conference. Some experts feel that they offer better aesthetics, too. Made with closed joints of mortar or sealant, cavity walls are weeped at the bottom and vented at the top, while the rainscreen variation uses open joints with a waterproofing membrane behind the outer material, enabling any moisture migrating behind the open panels to evaporate and letting water drain out from the bottom.
The BIA’s Trimble, who has served the masonry industry for the past 18 years, explains the cavity wall’s layers of moisture resistance as follows: “First, there is the outer skin, usually brick, which acts as a screen to rain and often resists most of the water. But if water does get in, there are other mechanisms to redirect the moisture. The air space is the next element that keeps the water from being transmitted through the wall. Gravity forces the water down to a system of flashing and weeps that let water out. If the air space is not effective, a water-resistant layer is used as a second line of defense.”
In terms of the drainage space itself, although masonry codes generally require one inch of clear air space within the cavity wall, the Brick Industry Association recommends a two-inch space as there is always a risk of mortar droppings bridging the space. While there are products and systems on the market that can be used to prevent mortar from collecting on the flashings, Gerns cautions that they not be used as a license for the mason to allow excess mortar to block the cavity.
Flashing at the veneer base. When installing the flashing system at the base of the veneer, Donaldson instructs that it be set at not less than 8-12 inches above finish grade, with seeps above the flashing. “Beneath the flashing, the masonry needs to be grouted solid to the supporting system to form a collar joint, or formed concrete should be used. Depending on what landscaping treatment is used adjacent to the building, the through-wall flashing may or may not lap over a waterproofing membrane to maintain consistency in a water/moisture infiltration barrier system,” he says.
Gubetta prefers working with membrane flashing, which he feels is an economical product that’s relatively easy to install. Key locations to focus on flashing include the base of cavity walls, above all openings, at all sill locations and continuous shelf angle locations, under copings and parapets, and at all level changes.
Because walls often slope, it’s important that the flashing be flexible and easy to form around bends, corners, and unusual shapes. In addition, because the base flashing is the final collection point for water draining through the wall, it should extend at minimum to the outside face of the masonry; ideally, it should extend past the outside face, according to Gerns.
He explains further: “In this instance, the flashing should have a metal drip edge or, better still, a metal sill pan. The base flashing should incorporate end dams at all transitions and terminations, and where laps occur, the laps should be sealed with at least three beads of sealant or butyl tape. The flashing system above the base flashing or the weather-resistant barrier (or both) should be integrated into the base flashing, such that any water which reaches the weather-resistant barrier drains to the base flashing and is discharged to the outside through regularly spaced weeps.”
In addition to flashing, sealing the intersections between the cavity and fenestration is also important, says Trimble: “It is easy to think about sealing out water or air in the middle of a wall, but when we get to windows and doors, it becomes much trickier to make the water- and air-barrier systems work.”
Trimble, who coordinates BIA’s educational activities and oversees publication of BIA’s technical documents, recommends visualizing the building enclosure in three dimensions when designing the water-resistant membrane and air barriers to seal the window and door openings. In many cases, drawing the detail in 3D helps not only the designer, but also the contractor who has to build it.
Gerns likes to work with cavity seals at the window penetrations through the cavity wall in order to prevent humid cavity air from hitting the curtain wall framing, which can cause condensation. While the type of seal is dependent on a number of factors—including the window detailing, the depth of the cavity, and the type of weather-resistant barrier—common materials include bituminous membrane, backer rod and sealant, spray polyurethane foam, extruded silicone, compressible foam, or a combination of these products.
Insulation factors. The thermal break line needs to be consistent between the exterior and interior as it runs between the veneer, openings, and the cavity wall system. To assist with this, Bostwick Design Partnership’s Donaldson advises wrapping the openings with a waterproofing membrane and then sealing the window or door to that membrane. With this approach, a secondary barrier is created to help restrict water infiltration through the opening’s perimeter.
Furthermore, additional insulation is recommended for the wall cavity, especially in metal-stud framing systems, to extend 18-24 inches beyond the edge of the opening to restrict the potential for compromising the thermal break, adds Donaldson.
Gubetta says he prefers to stay away from continuous cavity wall insulation and instead specifies rigid insulation, particularly mineral wool, as it doesn’t gum up the masonry veneer anchors and can be installed at any temperature. Donaldson, too, prefers rigid insulation board: “In recent projects, I have moved away from using batt insulation as I’ve found the rigid insulation board joints to be held tighter. The joints may tape, or if additional continuity is desired, sealed to one another. If more than one board can be utilized, staggering the joints between the layers of the rigid insulation board improves continuity.”
However, it’s important to be aware that even if one inch of rigid insulation is installed throughout the enclosure, this will not necessarily equate to a consistently strong R-value due to the wall penetrations and studs, which can lead to thermal bridging. To assist with this, Trimble recommends an ASHRAE 90.1 table which documents R-value based upon wall thickness and how close the studs are spaced.
Note that, when analyzing R-value, it is important to avoid the trap of overlooking other important design strategies, such as thermal mass. Because thermal mass is so effective at storing and efficiently using energy, the energy codes actually require less insulation in a wall with thermal mass, as opposed to a lighter-weight wall, according to Trimble.
In fact, a recent Portland Cement Association study (www.bdcnetwork.com/pacreport) found that walls with thermal mass are 14-21% more energy efficient than walls with minimal thermal mass. “That’s why whole building energy analysis is better than just talking R-values since the energy analysis programs can take into account thermal mass,” adds Trimble.
Lateral ties. Another construction problem can result from the incorrect installation of lateral ties, which anchor to the structural backup wall. While brick veneers are mostly known for their aesthetic and weather protection, it’s important to recognize the veneer’s potential to act as a structural component. Consequently, when masonry anchoring systems are properly installed, they effectively transfer lateral loads to the backup wall system.
“It is important that lateral ties not only provide adequate strength to resist applicable outward and inward loads, but that the ties also allow for the wall system to accommodate differential movement between the inner wall and the cladding system,” explains Gerns.
IKM’s Hartung also points out that conventional brick wall cavities consist of two inches of nominal air space and two inches of rigid insulation, so pre-engineered brick ties are made for these four-inch cavity walls. However, with a greater focus on energy efficiency, wall thickness has increased to three inches of insulation, creating a five-inch cavity. Fortunately, wall tie manufacturers have responded by providing larger-sized ties.
Expansion and contraction. In any veneer wall assembly, allowing for expansion and contraction is a major aspect of design detailing and construction methods, as Gerns explains: “To accommodate all these possible movements, expansion joints or control joints must be incorporated at regular intervals, at changes in materials, at changes in building massing, and at changes in geometry. The joints are typically installed vertically to accommodate horizontal movements and horizontally to accommodate vertical movements. The joint must be sized to accommodate initial movements and cyclical movements, and a sealant must be selected that can also accommodate the movement.”
It’s also important to note that, in its most recent literature, the Brick Industry Association has reduced the recommended spacing for masonry movement joints for walls with different types of masonry materials and a number of openings. The new BIA-recommended maximum spacing in walls with openings is approximately 20 feet; for walls without openings, the BIA-recommended spacing is about 25 feet.
“We have seen several projects lately with cracking developing within months after completion, where spacing in walls with openings was more than 30 feet, and it costs a lot of money and time to repair these walls,” cautions Gubetta.
On the single side of openings, the joints should be placed six feet or less; for openings greater than six feet in width, control joints should be provided on both sides of the opening, according to Donaldson. In addition, control joints should be installed in locations of brick relieving angles, because the veneer is more likely to move.
Because projects are unique, Donaldson recommends that design professionals collaborate with a structural engineer to understand the potential for building movement and what size of expansion control is recommended. For example, concrete shrinks over time, while clay brick masonry is more prone to expansion during the first few years as it absorbs atmospheric moisture. In general, wall systems either expand or contract based upon temperature variations.
MANUFACTURED STONE: THE NATURAL LOOK
While it’s tough to beat the natural look of stone, a number of manufactured stone products are coming real close to tricking the eye. As with any new building product, however, specifiers must do their homework and be forewarned that regardless of testing and research, time is the only true indicator of a product’s durability and performance.
Although ASTM created a subcommittee for adhered manufactured-stone masonry veneer a few years ago, their specification standard is still under development, so there is currently no consensus standard. “Most manufacturers have a variety of tests that they perform on their materials to make sure they meet a minimum level of durability, but until a consensus standard is developed, using product history is the best way to determine suitability and durability,” says Trimble.
Gerns points out that longevity is a critical consideration with newer products because deterioration due to loss of strength, change in appearance, or loss of other physical properties can result in premature failure of the material or system. When researching new product offerings, Hartung looks to company websites, social media, and knowledgeable product representatives to help make informed decisions.
Another useful tactic, applicable for all masonry systems, is the construction of scale and full-size mockups to most accurately judge how suitable any materials might be for a particular project. Mockups also benefit the Building Team by involving key trades and consultants to ensure every professional agrees with the method and outcome. “Relatively speaking, a mockup is a small investment for an owner and a designer to utilize in comparison to the large investment the owner is making in a project,” says Donaldson.
EVALUATING THE SUSTAINABLITY BENEFITD OF A BRICK WALL
In addition to the performance and aesthetic benefits proffered by brick veneer, good, old-fashioned bricks happen to be quite sustainable as well.Predominantly made from clay and shale, which are abundant natural materials, brick is commonly recycled for use as salvaged brick, chipped brick for landscaping, or crush brick for sub-base materials.
And although it does take energy to fire the brick kilns, more than 80% of brick manufacturers use natural gas or bio-based fuels such as sawdust or methane from landfills. Furthermore, because brick masonry has such a long service life—at least 100 years, according to the National Institute of Standards & Technology—the embodied energy over its life cycle is quite low.
Another area where brick shines is its thermal mass properties. Unlike other wall materials—such as vinyl, aluminum, wood, or EIFS—brick acts as an excellent medium to store heat and slowly release it, ultimately requiring less heating and cooling energy for the space.
Gerns also says that because façade systems are comprised of so many components from different manufacturers, it can be difficult to assess the details and integration of these materials and systems solely from shop drawings, or even from models. “Issues of constructability can often be identified and resolved [through the use of] mockups, and the aesthetics of the cladding system can be evaluated and modified, if deemed necessary,” he adds.
It is worth noting, however, that mockups do have their limitations as they are unable to fully simulate real-world conditions, such as areas of a wall system that may be prone to water infiltration. Consequently, field testing can be a much more accurate measure of such issues. Of course, this is an additional expense and should only be applied when necessary.
Ultimately, while it’s true that the design and installation of masonry systems requires a high level of skill, knowledge, and attention to detail, the payoff comes in the beautiful, classic look that can be achieved. BD+C
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EDITOR'S NOTE
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